Printing of relatively rigid sheet materials, e.g. made of corrugated cardboard or the like is known to be carried out using printing devices equipped with heads for printing a given surface of the material.
Particularly, these devices can be employed for the so-called large-format printing, as they can print large surfaces of sheet material, during continuous feeding of the latter.
For example, these devices are installed in plotters for limited edition printing, or in industrial printers designed for reel printing or stacks of folded material.
Typically, these heads are of ink jet type and may be electronically controlled to perform digital printing with continuous and very quick changes of printed designs while the material is being processed, as needed.
In cardboard-processing machines, for instance in customized package formation, there is a strong need to print the material almost over its entire width, as it is being continuously fed through the printing station.
A widely used arrangement to obtain such result is to install a plurality of fixed heads, offset over the width of the cardboard, such that the entire width of the latter may be covered by joining predetermined printing areas associated with respective heads.
US2008/0002011 discloses a method of manufacturing a corrugated cardboard product in which the printing step is carried out using an ink jet printer configured as described above.
Particularly, this printer comprises two fixed units, each composed of three ink jet heads arranged side-by-side in a corresponding transverse direction. The two units are longitudinally offset and placed above the cardboard to cover a printing area that is substantially equal to its entire width. The printer also has means for continuous feeding of the cardboard through the printing units, such that the latter are allowed to print the top surface of the material while being firmly secured to the frame of the printer.
One drawback of this prior art is the great number of heads required for printing the cardboard over its entire width, which becomes a dramatic limitation when printing large cardboard packages. Furthermore, this arrangement adds great complexity to the construction of the control unit of the printer, which is required to simultaneously control a great number of heads.
An attempt to obviate these drawbacks has been the development of printing units for cardboard or other similar sheet materials that have a smaller number of heads, the latter not being fixed, but moving along the transverse dimension of the material.
IT1394154, issued to the applicant hereof, discloses a device for printing sheet material, preferably for packaging use. This device has one or more printheads, arranged below the material feed plane to print the bottom face of the material, which forms the outer face of the package. The head is mounted to a transversely moving carriage, to sweep the entire width of the sheet material, for bottom-up printing of any area thereof.
While this arrangement is more versatile than fixed-printing unit configurations, it is still affected by drawbacks.
A first drawback of this arrangement is that the particular accommodation of the head below the material feed plane increases the risk of irregular bottom-up ink flow, thereby providing an imperfect printing image.
For optimal smearless, flawless printing, the head must be constantly supplied with a pressurized ink flow, and this is particularly difficult when upward ink ejection has to be promoted, against the force of gravity.
Furthermore the head receives ink from an external ink reservoir, which is connected thereto via a relatively long tube, which reduces supply pressure at the printhead nozzles.
An additional drawback of this prior art printing device is its complex assembly, due to need to manually connect the end of the ink supply tube to the sleeve attached to the bottom wall of the head, in a location that is hardly reachable by the operator.
Another drawback is that the head is secured to the carriage in cantilever arrangement with no particular protection, and is thus exposed to inadvertent impacts by various objects as it moves or during maintenance and cleaning.
Finally, replacement or removal of each head from the device is a time-consuming operation, because the operator is required to manually disconnect the ink supply duct and separate the head from the carriage in a narrow, hardly-accessible space.